Making accurate spectrum and power measurements on the desired portion of time-varying and complex modulated signals is a common measurement challenge. These measurements are made with a spectrum analyzer using one of several types of time-selective spectrum analysis, e.g. swept analysis or “gated local oscillator (LO)” or simply “gated sweep”. The sweeping local oscillator of the analyzer is directed to sweep and measure only during the desired portion of the signal. The goal is a measurement that has the same amplitude and frequency accuracy and resolution as a non-gated measurement, and which can be made quickly and simply. The task is complicated by the need for accurate triggering and gate timing, along with potential errors created by the interaction of the changing signal and the dynamic characteristics of the analyzer's LO and Intermediate Frequency (IF) filters. Three different techniques are generally used for time-selective spectrum and power measurements: Gated LO, Gated Video, and Gated Fast Fourier Transform (FFT).
For Gated LO measurements, the spectrum analyzer's local oscillator is controlled so that it is sweeping (and spectrum is measured) only during the desired portion of the input signal. Otherwise the measurement is halted or paused when the signal is outside of the gate interval, with sweeping resuming at the beginning of the next gate interval. Multiple sweep segments are required to generate a complete spectrum, with the number of segments required depending primarily on the analyzer's sweep time and the duration of the gate.
For swept spectrum analyzers, the gated LO technique is generally the best one to choose, if available on the analyzer you're using. Gated LO provides good flexibility in frequency span and gate timing, and measurements are completed faster than with gated video. Gated LO measurements are also easier to set up than gated video, as there is no need to carefully choose a sweep time. The transient responses within the analyzer are problematic. The transient response of the RBW and VBW (IF) filters reduces the available measuring time for each gate event and thus increases the number of segments required and thus the measurement time. The transient response of the LO adds measurement delay since the LO cannot be instantaneously started or stopped.
In implementing gated LO solutions in swept analyzers, the general practice has been to provide appropriate supplemental specifications for gated measurements, along with operational cautions and exceptions. Whether time-gated measurements are specified or not, in some cases spectrum analyzer users are unaware of the potential accuracy and repeatability problems of time-gated measurements.
The most straightforward way to account for the settling time required by the IF filters and preselectors is to add a delay (from when the gate opens and/or the signal is valid) to the measurement process. To improve measurement speed and allow for the shortest gate times, this delay should be minimized. Two sources of error remain. First, data acquired at the end of a sweep segment when the LO is settling to a stop: The frequency of some measured data will be distorted by the frequency settling of the LO as it is stopped at the end of the gate. Second, sweep linearity errors (frequency and span errors) accumulate due to small LO frequency errors in each sweep segment: The usual practice of sweeping for precisely the duration of the gate does not allow for any correction of imperfect frequency start/stop values associated with non-ideal behavior of the LO at the beginning and end of gate segments.
For a first order solution the starting and stopping of the analyzer's LO must be accurately controlled, and the sweeping must remain as linear as possible despite the start/stop operations. Since the spectrum result is assembled from multiple sweep segments, each one must accurately match those before and after it, and timing/frequency errors must not be allowed to stack up, especially in situations where a spectrum measurement is made from a large number of segments.
For Gated Video measurements, the analyzer sweeps continuously with the gate signal used to select which measurement points are retained and displayed. The analyzer's sweep time must be set so that the gate signal is valid during at least some portion of the duration of each display point, or gaps in the measured spectrum will result. Thus gate time must be carefully chosen, and hundreds of gate events may be required for a complete spectrum measurement, frequently resulting in slow total measurement times.
For Gated FFT measurements, the signal in the analyzer's IF is digitally sampled and a time record is constructed from the samples that are within the gate window. A Fourier transform is performed on the time record, yielding a spectrum measurement of the signal from the valid gate interval only. No sweeping local oscillator is involved, and in many cases the spectrum can be measured from a single gate interval. The gated FFT mode does not provide adjustable resolution bandwidth, and measurements requiring concatenated FFTs will be much slower than those that do not.
Understanding the effective accuracy of a particular gated measurement is difficult because of the interaction of the signal and the analyzer's gated measurement hardware and firmware. For example, two different measurements using the same analyzer setup (gate timing, span, etc.) might have different errors due to differing spectral content or power level of the signal under test.